Energy from Reactions
Calculating the energy changes in chemical reactions can be a little worrying but the only maths skills required are addition and subtraction ... you can slip in a little multiplication if you wish!! The only chemical skill you need is to be able to work out what bonds are present in each substance but in the GCSE Chemistry exam, there will be plenty of clues in the question.
It pays to be organised, even in the exam. Take a sheet of paper and draw a vertical line down the centre. The left column will be for the reactants and the right for the products. Start by writing down the individual bonds in the reactants. Next, write down how many of each bond there is. Then write down the bond energy for each one and calculate the energy contained in that number of each bond. Then work out total energy contained in the bonds of the reactants by adding the individual totals together. Do this step by step and double check at each stage to eliminate any errors.
Repeat the process for the products. Then subtract the total bond energy of the reactants from the products, paying careful attention to whether the answer is a positive or a negative number. Positive numbers mean that the reactions are endothermic, negative numbers mean that they are exothermic chemical reactions.
The questions that follow concern making and breaking bonds and the energy changes involved in doing so. To be able to complete these questions you will need the following bond energy information and a calculator:
- C-H 413kJ/mol
- O=O 498kJ/mol
- C-O 358kJ/mol
- O-H 464kJ/mol
- N≡N 945kJ/mol
- H-Cl 432kJ/mol
- Cl-Cl 243kJ/mol
- H-H 436kJ/mol
- C=O 805kJ/mol
- C-C 347J/mol
- N-H 391kJ/mol.
Now try these ten questions on energy changes - good luck!
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